Turning circuits are frequently used in feedback configurations where it is required that a signal from a network be balanced against a reference parameter. Such tuning circuits commonly utilize operational amplifiers as the driving element to balance the circuit. It has heretofore been the usual practice to design an operational amplifier circuit having differential inputs such that the common mode input voltages are at approximately the midpoint between the power supply voltages applied to the operational amplifier. It is a feature of certain operational amplifiers that the leakage current through the amplifier is proportional to the common mode input voltage. In tuning circuit applications which require long term stability, this leakage current can cause the circuit to drift away from its balance condition. One application of this type is in metal detector circuits wherein the circuit is balanced for a given set of operating conditions and then the detector is operated for a period of time under these conditions. If the leakage current through a metal detector tuning circuit used is significant, the circuit will drift out of balance and require frequent retuning.
Therefore, there exists a need for a tuning circuit which reduces the leakage current associated with an operational amplifier to thereby provide a low drift rate and long term stability.